1. Field of the Invention
This invention relates to a responder or a transponder in a movable-object identification system.
2. Description of the Prior Art
There are known movable-object identification systems in which a responder (a transponder) mounted on a movable object can communicate with a fixed interrogator by radio. When the responder receives an interrogation signal (a challenge signal) from the interrogator, the responder automatically transmits a reply signal including identification information related to the movable object.
In some cases, an identification-code generator of a responder includes a memory storing various pieces of identification information, and the identification-code generator consumes an appreciable rate of dc power.
Some responders derive dc power from radio wave energy induced in an antenna. Specifically, such a responder has two antennas. One of the antennas is used for deriving dc power. A received signal induced in the other antenna is used as a carrier for a transmission signal. The two-antenna design is disadvantageous in compactness.
Japanese published unexamined patent application 1-218965 discloses a responder having a single antenna. In the responder of Japanese application 1-218965, a received interrogation signal induced in the antenna is divided by a distribution device into two, one being used for starting a CPU while the other being used as a carrier for a transmission signal. Specifically, the responder of Japanese application 1-218965 includes a diode for subjecting the part of the interrogation signal to a detection process, and a comparator for converting the level of the output detection signal from the diode into a binary CPU start control signal. The distribution device generally occupies a considerable space, so that the transponder of Japanese application 1-218965 tends to be large.